1. Technical Field
The present invention relates to a transmission apparatus and a method of controlling the same, and more specifically, to a transmission apparatus and a method of controlling the same that are capable of protecting a power amplifier included in the transmission apparatus from a reflected wave.
2. Background Art
A transmission apparatus used in a radio system and the like includes a power amplifier of high-frequency and high-output. This power amplifier may be destroyed by a reflected wave counterflowing from a circuit such as a filter provided in an output side of the power amplifier, for example. Related art techniques for solving such a problem include the following techniques.
The first technique is the one for detecting, in an output side of a power amplifier, a reflected wave or return loss (voltage standing wave ratio) that is caused by inconsistency in the output side of the power amplifier, thereby preventing the reflected wave from counterflowing and the power amplifier from being destroyed using a protection circuit when the detection result is beyond an acceptable level.
The second technique is the one for directly detecting whether a transmission frequency of the transmission apparatus is within a predetermined bandwidth, and for example, interrupting a power supply of the power amplifier when the transmission frequency is out of the predetermined bandwidth, thereby preventing the power amplifier from being destroyed.
The first technique will be described first. FIG. 9 is a circuit diagram showing a protection circuit of a power amplifier disclosed in Japanese Unexamined Patent Application Publication No. 2005-191791. In the protection circuit of the power amplifier shown in FIG. 9, a directional coupler 104 is provided in an output side of a power amplifier 102, and an increase in the reflected wave power from a load is detected in a power detector 106 through the directional coupler 104. Then, by supplying a detection signal of the power detector 106 to a power supply voltage control circuit 109, the power supply voltage supplied to the power amplifier 102 is gradually reduced. Hence, the power supply voltage supplied to the power amplifier 102 can be gradually reduced as the power amplifier 102 approaches the breakdown condition. Hence, it is possible to prevent the power amplifier 102 from being destroyed due to the change of operating conditions of the power amplifier such as load variations.
Further, in a transmission circuit disclosed in Japanese Unexamined Patent Application Publication No. 2000-341145, a directional coupler is provided between a power amplifier and an antenna. Then a reflected wave that reflects in the side of the power amplifier from the side of the antenna is detected with the directional coupler, and the supply of the power supply voltage applied to the power amplifier is interrupted when the level of the reflected wave is equal to or more than a predetermined value. Accordingly, it is possible to prevent the destruction of the power amplifier by the reflected wave without using a high-priced isolator.
Further, in a transmission circuit disclosed in Japanese Unexamined Patent Application Publication No. 2005-051349, an isolator is interposed between a power amplifier that receives a transmission signal and a transmission antenna, a bypass forming unit is provided between an input side and an output side of the isolator, and a power monitor circuit monitors the reflected power reflected by the transmission antenna. When the reflected power is equal to or below a reflected power reference value, a bypass control circuit closes the bypass forming unit (switch) to shunt the isolator. Meanwhile, when the reflected power exceeds the reflected power reference value, the bypass forming unit (switch) opens and the isolator is inserted between the power amplifier and the transmission antenna.
Further, a power amplifier disclosed in Japanese Unexamined Patent Application Publication No. 11-284522 includes a power amplifier unit that amplifies a high-frequency signal, a coupling unit that extracts a part of power sent from the power amplifier unit to an antenna, and an amplification controller that uses the extracted power for controlling the power amplifier unit. The power amplifier unit includes a coupler that connects each of a travelling wave and a reflected wave to the coupling unit, and controls attenuation or interruption of a transmission output of a transmission power amplifier or a transmission buffer amplifier using a voltage standing wave ratio calculated from travelling wave power and reflected wave power obtained from the coupler.
Next, the second technique will be described. FIG. 10 is a block diagram showing a microwave amplification device disclosed in Japanese Unexamined Patent Application Publication No. 06-152265. The microwave amplification device shown in FIG. 10 includes amplifier circuits 201 and 202, a power supply 204, a microwave transmission line 206, out-band signal detection circuits 207 and 208, and a control circuit 209. In the microwave amplification device shown in FIG. 10, the amplifier circuits 201 and 202 are controlled according to a control signal 205 using the control circuit 209, and the amplifier circuits 201 and 202 perform impedance conversion with the operation frequency bandwidth. When the microwave which is out of the operation frequency bandwidth is input, this is detected using the out-band signal detection circuits 207 and 208, and the control circuit 209 stops the operations of the amplifier circuits 201 and 202.
Further, a technique related to a transmission output control method is disclosed in Japanese Unexamined Patent Application Publication No. 2001-168740. A radio device according to Japanese Unexamined Patent Application Publication No. 2001-168740 includes a frequency conversion means that converts an intermediate frequency signal to a radio frequency signal, an amplifier means that amplifies the radio frequency signal to generate a transmission signal, and a power supply for amplification means that generates a power supply voltage for the amplification means. Further, the radio device includes a local oscillation means that generates a local oscillation signal of the frequency conversion means while stabilizing its frequency and detects frequency abnormality of the local oscillation signal, a device power supply means that supplies a power supply to in-device components, a device power supply output detection means that detects an output of a device power supply, and a transmission output control means that reduces the voltage of the power supply for amplification means or interrupts the power supply for amplification means upon detection of at least one of frequency abnormality and output abnormality of the device power supply.
A technique related to a transmission apparatus that prevents transmission of an unwanted wave due to abnormality of a local oscillator is disclosed in Japanese Unexamined Patent Application Publication No. 11-251935. FIG. 11 is a block diagram showing a transmission apparatus disclosed in Japanese Unexamined Patent Application Publication No. 11-251935. The transmission apparatus disclosed in Japanese Unexamined Patent Application Publication No. 11-251935 includes a frequency converter (MIX) 301, a local oscillator 302, a controller (CONT) 303, and a switch (SW) 304. The frequency converter (MIX) 301 outputs a high-frequency signal RF obtained by mixing an input signal IF and a local oscillation signal LO and converting the frequency. The local oscillator 302 outputs a local oscillation signal. The switch (SW) 304 is provided between the frequency converter 301 and the local oscillator 302, and prevents the local oscillation signal LO from being supplied to the frequency converter 301. The controller (CONT) 303 controls the switch 304 upon detection of abnormality in the local oscillator 302, so as to interrupt the local oscillation signal supplied to the frequency converter 301.